1. Field of the Invention
The present invention relates to an image forming apparatus that uses an electrophotographic method, and in particular relates to an image forming apparatus that includes an intermediate transfer belt or an electrostatic conveyance belt.
2. Description of the Related Art
With an electrophotographic image forming apparatus, a so-called tandem type is known where an image forming unit is provided individually for each color in order to print at a high speed. A tandem image forming apparatus having an intermediate transfer belt successively transfers images from the image forming units for the respective colors onto the intermediate transfer belt and furthermore transfers the images all at once from the intermediate transfer belt onto a recording medium. Also, with a tandem image forming apparatus having an electrostatic conveyance belt, images are transferred from the image forming units for the respective colors onto a recording medium on an electrostatic conveyance belt. Note that in the description below, “belt” is used to refer to the intermediate transfer belt and the electrostatic conveyance belt collectively.
With this type of image forming apparatus, color misregistration (misregistration) can occur when the images are overlaid on each other due to mechanical factors in the image forming units for the respective colors. For example, the belt or a roller driving the belt expands/contracts due to temperature change inside of the image forming apparatus, and the movement speed of the belt surface (hereinafter referred to as the belt speed) changes. Due to the change in belt speed, a shift occurs in the position at which the images are overlaid, which causes color misregistration.
Because of this, Japanese Patent Laid-Open No. 2003-233234 discloses a configuration where misregistration is controlled by installing a sensor for detecting the rotation speed of a driven roller that is driven by the rotation of the belt and performing rotation speed control for the driving motor of the belt such that the rotation speed of the driven roller is a constant speed.
However, as described above, in order to provide a sensor for detecting the rotation speed of the driven roller of the belt, it is necessary to secure an installment position for the sensor, which will cause the size of the image forming apparatus to increase, and the cost will furthermore increase.